Aircraft windows must satisfy several requirements. Once such requirement is a clear visibility from the cabin to the outside. Further, the window structure must be capable of maintaining the static pressure inside The cabin even at high altitudes. Window components are not supposed go protrude outside the outer skin surface of the aircraft body in order to avoid an adverse influence of the window construction on the aerodynamic characteristics of the outer aircraft skin to avoid any increase in the drag caused by the window construction.
Conventional aircraft windows comprise two window panes for an increased reliability. One of these window panes, namely the outside main pane, is relatively thick and has a high fatigue strength achieved for example by making the thick window pane of stretched acrylic glass. The other window pane is relatively thin and has a small fatigue strength. Such thin window panes are made of stretched acrylic glass. Such thin aircraft window panes are referred to as auxiliary panes. Conventionally, the thick or main window pane is installed As an outer window pane in a window frame away from the cabin while the thin auxiliary pane is installed ac an inner pane next to the cabin. The construction is conventionally such that the strong outer window pane takes up the cabin interior pressure in normal operating condition. The inner auxiliary pane is not required to take up the inner cabin pressure under normal operating conditions since the cabin pressure is effective between the inner and outer window panes. Both panes are held in a window frame by an elastic sealing profile that holds both panes in the frame as a structural, nodular unit which is inserted into the window frame and secured in the frame which in turn is secured to the aircraft body structure.
As mentioned, the space between the panes is conventionally connected to the interior of the cabin through a small diameter bore so as to provide a pressure equalization between the space between the panes and the interior of the cabin. It the exterior main pane should break, it is necessary that the auxiliary pane takes up the entire anterior cabin pressure. However, in such an emergency it is only necessary to assure that the particular flight can continue to its destination without a total failure of the inner window pane. On the other hand, the main window pane must be capable of taking up the inner cabin pressure under all operating conditions, including normal operating conditions. As a result, the main pane has a tendency to bulge outwardly, whereby the stretching of the material facilitates the adverse effects of external harmful materials such as corrosives becoming effective on the outer window surface. The effects of corrosives and the like aided by the outward bulging of the main window pane are thus very disadvantageous, especially with regard to the useful service life of the main window panes which are rather expensive and must be frequently replaced when they no longer permit a clear visibility to the outside. Moreover, the outward bulging of the main window panes in conventional aircraft windows adversely affects the aerodynamic drag, thereby increasing the drag. Another disadvantage is seen in that the connection of the space between the panes to the cabin facilitates fogging of the window panes when moist air enters into the space between the panes so that the visibility through the window is impaired.
German Patent 693,159 (Wagner et al.), published on Jun. 6, 1940 discloses a self-supporting transparent nose cone shell structure of an aircraft. A transparent, strength providing inner wall (2) of the nose cone carries a plurality of radially outwardly extending ribs (18) circumferentially distributed around the nose cone for carrying an outer transparent wall skin (3). The space between the two transparent walls (2) and (3) is heated to prevent fogging of the nose cone.
German Patent 737,294 (Diez et al.), published on Jun. 3, 1943 discloses an aircraft cabin window with a stronger outer window pane (1) and a thinner inner window pane (2). The space (4) between the window panes is sealed by a seal (3) and an air cleaner device is positioned to reach into the inner space. The air cleaner device holds, for example active charcoal or silica gel.
German Patent 933,371 (GOtz), published on Aug. 25, 1955 discloses a dryer cartridge for an aircraft window that is inserted through the thinner inner window pane into the space between the inner pane (9) and the outer stronger pane (10). A tablet (3) of air drying material is inserted into the cartridge which is also equipped with a rubber membrane that responds to a pressure difference between the pressure in the space between the panes and the cabin pressure. The cartridge is exchangeable.
German Patent Publication 1,252,533 (Hertel), published on Oct. 19, 1967 discloses aircraft cabin windows that have a common frame structure which in turn forms part of the aircraft body structure. A strong inner pane (6) and a smooth outer covering (9) enclose a space (3). The outer covering (9) is supposed to reduce aerodynamic drag.
East German Patent Publication 23,321 (Riedel et al.), published on Jun. 29, 1962 discloses a double window for pressure chambers, especially aircraft cabins in which in addition to the regular window frame that spaces the stronger inner window pane (4) from the thin outer window pane (7), a further frame (8) is provided which holds a foam rubber seal (13).
The just described prior art leaves room for improvement, especially with regard to the arrangement of the window panes and with regard to the control of the air pressure in the space between the window panes.